Semiconductor waveguide structures are well known. For instance, semiconductor lasers typically are designed to comprise a waveguide structure that confines the radiation in the direction normal to the plane of the layers. Such a structure generally comprises a first and a second cladding region, with a core region between the cladding regions. The layer structure of the laser is selected such that the refractive index of the core region is greater than the refractive indices of the first and second cladding regions, respectively. The refractive indices of the first and second cladding regions need not be the same.
Semiconductor waveguide structures can readily be designed and made for conventional III/V devices. Such devices typically have operating wavelengths .ltorsim.2 .mu.m. Recently a novel device, the quantum cascade (QC) laser, was disclosed. See, for instance, J. Faist et al., Applied Physics Letters, Vol. 66(5), p. 538 (1995). See also U.S. patent application Ser. No. 08/223,341. The QC laser will generally be implemented in III/V technology and can be designed to emit radiation of wavelength substantially longer than that emitted by conventional III/V lasers. For instance, the above cited paper describes a QC laser that operated at about 4.5 .mu.m. In principle, QC lasers can operate at even larger wavelengths. However, our initial attempts to design a QC laser that emits radiation of wavelength .gtorsim.5 .mu.m revealed the existence of a problem that was, to the best of our knowledge, not previously encountered.
As can be seen from Table I of the above referenced U.S. patent application, an exemplary 4.2 .mu.m QC laser had an upper cladding region consisting of a 1 .mu.m thick n-doped (1.5.times.10.sup.17 /cm.sup.3) AlInAs layer and a 1.5 .mu.m n-doped (5.times.10.sup.17 /cm.sup.3) further AlInAs layer, with the total layer structure being of approximate thickness 5 .mu.m. Deposition of such a relatively thick layer structure by MBE (or other deposition methods, e.g., CBE, that may be suitable for making a precision layer structure such as is required for QC lasers) is time consuming.
As will be readily understood by those skilled in the art, the required thickness of the cladding layer of a semiconductor waveguide structure generally increases with increasing wavelength. It will also be understood that the presence of a metal contact layer on a cladding layer imposes stringent limits on the permitted strength of the optical field at the metal/semiconductor interface, if losses of the optical field are to be kept to an acceptable level. These considerations point to an impractically large upper cladding layer thickness in QC lasers that emit at wavelengths .gtorsim.5 .mu.m.
We have invented a novel waveguiding structure that is suitable for use in long wavelength (typically .gtoreq.5 .mu.m) QC lasers, and that can also be advantageously used in other long-wavelength devices that comprise a semiconductor waveguide structure, e.g., in an optical modulator. As will be explained below, the novel structure utilizes an effect that was, to the best of our knowledge, not previously relied on in waveguide structures. Use of the effect makes possible structures that have a substantially thinner cladding layer than would be necessary in prior art technology.